Phase plug

In a loudspeaker, a phase plug, phasing plug or acoustical transformer is a mechanical interface between a speaker driver and the audience. The phase plug extends high frequency response because it guides waves outward toward the listener rather than allowing them to interact destructively near the driver.

Phase plugs are commonly found in high-powered horn loudspeakers used in professional audio, in the mid- and high-frequency bandpasses, positioned between the compression driver diaphragm and the acoustic horn. They may also be present in front of woofer cones in some loudspeaker designs. In each case they serve to equalize sound wave path lengths from the driver to the listener, to prevent cancellations and frequency response problems. The phase plug can be considered a further narrowing of the horn throat, becoming an extension of the horn to the surface of the diaphragm.

An electromechanical driver of the sort later used in loudspeakers was invented by German industrialist Werner von Siemens in 1877 but no practical amplification existed to create the loudspeaker until 1921. Various loudspeaker designs were produced in the 1920s, including General Electric engineers Chester W. Rice and Edward W. Kellogg mating an acoustic horn to the speaker driver in 1925. In 1926, Bell System engineers Albert L. Thuras and Edward C. Wente modified the horn loudspeaker by inserting the first phase plug between the driver and the horn. This phase plug directed sound waves into the horn throat from the center of the diaphragm and from a ring around the perimeter of the diaphragm, by way of center hole and annular slot, for the purpose of improving "the transmission characteristics" of the loudspeaker "at the upper portion of the sound frequency range." Based on their joint research, the two engineers were awarded consecutive US patents: Thuras filed a patent for a novel electrodynamic diaphragm design, and Wente filed a patent for the first phase plug. The principles laid out by Thuras and Wente have influenced every subsequent phase plug design.

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